It ran too long, the characters got "flanderized" to the point of no return, they can phone it in because the ratings are always solid.

And let's face it, if they ended it Fox will lose a crap ton of viewers.They rarely play the classic episodes of Simpsons now, fuck I remember they would play them back to back weekdays at 6. Now they only play them once a day and it's a crappy newer one.